Campfield and Houde: Ichthyoplankton community structure and comparative trophodynamics 
9 
Component 
1 
2 
3 
4 
5 
Variance 
0.39 
0.21 
0.13 
0.09 
0.06 
Loading 
-1.65 
0.86 
0.91 
-1.95 
2.18 
Component 
1 
2 
3 
4 
5 
Variance 
0.43 
0.16 
0.11 
0.09 
0.07 
Loading 
-0.13 
-1.33 
0.15 
1.20 
0.11 
Figure 6 
Plots of ichthyoplankton species within the first two components of principal components analysis, and loadings and vari- 
ances explained by each component in year 2000 (at left) and year 2001 (at right). See Appendix I and II for full species 
names. Focal species of this study are shown in bold. 
significant among large (>10-mm) white perch and 
striped bass larvae. 
Discussion 
Estuarine transition zones in coastal plain estuaries are 
regions of strong hydrographic gradients that control 
distributions of icththyoplankton and zooplankton and, 
potentially, their trophic interactions. We propose that 
the larval assemblage structure, zooplankton distribu- 
tions, and trophodynamics observed in the Patuxent 
River provide insight into the role of salt front-ETM 
features and dynamics in other temperate estuaries that 
receive variable freshwater inputs and which support 
fishes with diverse life histories (estuarine-dependent, 
anadromous, and freshwater resident). We documented 
interannual variability of the ichthyoplankton assem- 
blages in the estuarine transition zone of the Patux- 
ent River in 2000 and 2001. Two larval assemblages, 
riverine and estuarine, were identified in each year. In 
2000, abundance of ichthyoplankton was highest in the 
oligohaline region immediately below the salt front, but 
in 2001 highest abundance was at the salt front. Our 
two hypotheses were supported: 1) environmental gra- 
dients defined distributions and assemblages of larvae, 
and 2) the salt front was a factor controlling assemblage 
structure and trophic interactions. 
Larval assemblages and environmental factors 
Moronid and alosine larvae occurred in fresh to oligoha- 
line waters of the Patuxent River subestuary. Although 
moronids are commonly found in salinities of 0-3 (North 
and Houde, 2001), the distribution centers of alosine 
larvae occur in freshwater further up-estuary (Setzler 
et ah, 1981; Bilkovic et ah, 2002). In addition to inter- 
annual differences in overall distributions, ontogenetic 
migrations or shifts are common, resulting in species 
distributions that are most discrete during the earliest 
larval stages and greater overlaps in taxa distributions 
during ontogeny. For example, larvae of alewife at all 
stages were confined to the freshwater region, and small 
naked goby larvae were confined to the oligohaline 
region. However, during ontogeny naked goby larvae 
dispersed up-estuary and its late-stage larvae were 
found with alewife in freshwater. 
A salt front and estuarine turbidity maximum (ETM) 
often characterize coastal plain estuaries. In Chesa- 
peake Bay, they typically occur in the 0-3 salinity 
region of the mainstem Bay and its tributaries (North 
and Houde, 2001, 2003; Sanford et al., 2001). As larval 
fishes develop, they may converge at the salt front- 
ETM through passive transport and retention related 
to hydrographic and circulation features. Convergence 
also could be facilitated by active tracking by larvae 
of elevated zooplankton prey concentrations, especially 
